1. Field of the Invention
The present invention relates to an image processing circuit which encodes image data and a liquid crystal display apparatus including the image processing circuit.
2. Related Art
BTC technology for image compression has been proposed. A three level BTC has been proposed which compresses image data in a pixel block consisting of multiple pixels at three levels (see reference 1: Alsaka & Lee 1990) Y. A. Alsaka, D. A. Lee. “Three Level Block Truncation Coding,” IEEE, Proceedings 1990 Southastcon, p. 421). The technique described in this reference fixes thresholds “th” and “tl” where “th=MAX−R/3” and “tl=MIN+R/3” and fixes representative values “a” and “b” classified by the thresholds “th” and “t1” at “a=(th+MAX)/2” and “b=(tl+MIN)/2” based on a range “R=MAX−MIN” determined from a maximum value “MAX” and minimum value “MIN,” without considering distribution of pixel values. PC images, if compressed by this technique, may cause visible artifacts.
Another three-level BTC has been proposed (see reference 2: Efrati 1991] N. Efrati, H. Liciztin. “Classified block truncation coding-vector quantization: An edge sensitive image compression algorithm,” Signal Processing: Image Communication 3 (1991) 275-283, Elsevier Science Publisher). The technique described in this reference determines three-level BTC thresholds using representative values “a2” and “b2” determined by two-level BTC in advance. For instance, it determines two thresholds “t1” and “t2” by performing calculations “t1=(3×b2+a2)/4” and “t2=(3×a2+b2)/4,” classifies image data into three groups using the thresholds “t1” and “t2,” and determines average values of the respective groups as their representative values. Using these representative values as initial values, the technique repeats calculations to determine optimum thresholds and representative values.
Again, since the thresholds and representative values are determined in a fixed manner, visible artifacts may be caused especially in the case of PC images.
Yet another three-level BTC has been proposed (see reference 3: Mor 1992] I. Mor, Y. Swissa, H. B. Mitchell. “A fast nearly optimum equi-spaced 3-level block truncation coding algorithm,” Signal Processing: Image Communication 6 (1994) 397-404, Elsevier Science Publishers). The technique described in this reference generates three levels at equal intervals, centering on a representative value of image data. The two thresholds “t8” and “t2” are given by “t1=c−s/2” and “t2=c+s/2.” It performs calculations repeatedly by optimizing mean square error (MSE) as an objective function, and thereby determines final thresholds.
Since this technique must perform calculations repeatedly until final thresholds are determined, there is a problem that the calculation is computationally very intensive.
Any of the techniques described above has the problem of visible degradation for PC images processed by it. False colors may appear in relatively large areas, especially in window frames, banners, error display panel portions, input prompt portions, and the like which have high black-and-white contrast and in which pixel data having three values or more are neighboring each other including a background and characters. Also, three-level BTC has a lower compression ratio than two-level BTC because the number of representative values increases from two to three.